1. Field of the Invention
The present invention relates to an implantable medical device.
2. Description of the Prior Art
In an implantable medical device, such as a cardiac pacemaker, an implantable cardioverter/defibrillator, or an insulin dispenser, telemetry is used e.g. to change or modify operation characteristics of the implantable device or to read out data from the implantable medical device to monitor its functioning or to obtain information about the patient in whom the device is implanted. Telemetry systems for implantable medical devices have utilized radio-frequency energy to enable communication between the implantable device and an external programmer device.
Earlier telemetry systems used a rather low radio frequency, i.e. 13.5 MHz, as a carrier wavelength for communication between an antenna of the implantable device and an antenna of the external programmer device, which were inductively coupled to each other. Due to the very poor operating distance of this technique, the exterior antenna had to be located in close proximity to the implantable device, typically within a few inches. Further, the communication suffered from low transmission data rate.
Recently, telemetry systems using a radio frequency data link operating at a much higher frequency, around 400 MHz, have been proposed, which enable two improvements to be made. First, the antenna efficiency can be improved allowing the range between the pacemaker and the external antenna to be extended. Second, the transmission data rate can be improved. Even higher frequencies can be used such as those within the ISM-band at 2400–2485.5 MHz.
U.S. patent application Publication 2002/0095195 discloses an implantable medical device utilizing such far-field electromagnetic radiation to allow communication over a large distance. Two conductive halves of a housing for the implantable device act as a dipole antenna for radiating and receiving far-field radio frequency radiation modulated with telemetry data. The conductive halves are separated by an insulating header, in which conducting leads can be located.
Although U.S. patent application Publication 2002/0095195 discloses a manner to utilize the limited space for the antenna function, but, nevertheless, there are several limitations as to the use of an implantable medical device in which two separated conductive halves of the housing act as a dipole antenna.
First, because the header, made of dielectric material, is disposed between the two conductive halves, this allows external interfering radiation to enter the implantable device and interfere with signals transmitted within any of the two conductive halves or with signals transmitted across the dielectric header.
Second, in order to hermetically seal the two housing halves, a number of feed-throughs between them are needed since different electric circuitry is located in different housing halves to effectively use the available space.
Third, the design of the implantable medical device does not allow for an optimum location of the conducting leads with respect to possible interference of the therapy signals by radio frequency signals fed to or received by the dipole antenna. The antenna type lacks a voltage node on its surface, where the electric field has a minimum, so the therapy signals would not have been obvious to a person of ordinary skill in the would be affected to a minimum extent.
Finally, the mechanical structure of this known implantable medical device seems not to be optimum, because two housing portions of a conductive material have to produced, and must be fixed to and hermetically sealed against an intermediate piece of dielectric material. The manufacturing is further complicated by the need for a number of feed-throughs.